Flash memory is one type of non-volatile memory that can be electrically erased and reprogrammed. Flash memory is erased and programmed in blocks where each block includes multiple memory cells. Each memory cell includes a floating gate transistor for storing information. Each floating gate transistor stores one or more bits of data. One type of flash memory is NAND flash. NAND flash uses tunnel injection for writing data to a memory cell and tunnel release for erasing data from a memory cell. NAND flash memories are accessed as block devices. The blocks are typically 512, 2048, or other suitable number of bits in length.
Another type of non-volatile memory is charge trapping memory, specifically semiconductor-oxide-nitride-oxide-semiconductor (SONOS) memory. A SONOS memory cell typically includes a standard metal-oxide-semiconductor (MOS) transistor with a charge trapping dielectric. The charge trapping dielectric is where the data bit or bits are stored. The charge trapping dielectric typically includes a multilayer structure including a first oxide layer, a nitride layer, and a second oxide layer. With the gate biased positively, electrons from the emitter-collector circuit tunnel through the first oxide layer and get trapped in the nitride layer. The trapped electrons provide an energy barrier between the emitter and the collector that raises the threshold voltage (Vt) of the transistor, which is used to store bits of data. The electrons are removed by applying a negative bias on the gate.
After programming a memory cell, the state of the memory cell can be sensed by applying a small gate voltage to the storage transistor to be read. The voltage is selected to be between the threshold voltages expected for the states to be sensed. If current flows between the collector and the emitter, the memory cell is in the no trapped electron state, and is therefore set to a logic “0” state. If no current flows between the collector and the emitter, the memory cell has trapped electrons and is therefore set to a logic “1” state. Similarly, several different Vt states can be used to store multiple bits of data.
One type of transistor is a finFET transistor. A finFET transistor is a metal-oxide-semiconductor field effect transistor (MOSFET) multi-gate transistor. A finFET transistor is typically fabricated on a silicon on insulator (SOI) substrate. The gate of the transistor is placed on at least two sides of the channel of the transistor to form a multi-gate structure and hence provides improved channel control.
To increase the storage density of the NAND flash, the minimum feature size (F) is reduced. To reduce the minimum feature size requires increasingly expensive lithography and processing tools to perform complex processing. In addition, the intrinsic scaling of the transistors as well as the capacitive coupling of the floating gates is reaching physical limitations.
For these and other reasons, there is a need for the present invention.